fix(toggleButtonGroup): color and border don't overflow now

[maur0xis]

By adding box-sizing: border-box; the component gets the correct behavior. Fixes #91

Current look:

tested with yarn link

[Tigge]

This changes the appearance a bit, which we don't want:

I just realized this is due to us not setting box-sizing: border-box;. Commonly * { box-sizing: border-box; } is used in modern projects. That is what we are using in the documentation as well. But that should not be required.

[maur0xis]

I just realized this is due to us not setting box-sizing: border-box;.

Added